1. Field of the Invention
The present invention relates to a system and a method for thermal management of an electrically stimulated semiconductor integrated circuit undergoing probing, diagnostics, or failure analysis.
2. Description of the Related Art
Integrated circuits (ICs) are being used in increasing numbers of consumer devices, apart from the well-known personal computer itself. Examples include automobiles, communication devices, and smart homes (dishwashers, furnaces, refrigerators, etc.). This widespread adoption has also resulted in ever larger numbers of ICs being manufactured each year. With increased IC production comes the possibility of increased IC failure, as well as the need for fast and accurate chip probing, debug, and failure analysis technologies. The primary purpose of today""s probing, debug, and failure analysis systems is to characterize the gate-level performance of the chip under evaluation, and to identify the location and cause of any operational faults.
In the past, mechanical probes were used to quantify the electrical switching activity. Due to the extremely high circuit densities, speeds, and complexities of today""s chips, including the use of flip-chip technology, it is now physically impossible to probe the chips mechanically without destructively disassembling them. Thus, it is now necessary to use non-invasive probing techniques for chip diagnostics. Such techniques involve, for example, laser-based approaches to measure the electric fields in silicon, or optically-based techniques that detect weak light pulses that are emitted from switching devices, e.g., field-effect transistors (FETs), during switching. Examples of typical microscopes for such investigations are described in, for example, U.S. Pat. Nos. 4,680,635; 4,811,090; 5,475,316; 5,940,545 and Analysis of Product Hot Electron Problems by Gated Emission Microscope, Khurana et al., IEEE/IRPS (1986), which are incorporated herein by reference.
During chip testing, the chip is typically exercised at relatively high speeds by a tester or other stimulating circuit. Such activity results in considerable heat generation. When the device is encapsulated and is operated in its normal environment, various mechanisms are provided to assist in heat dissipation. For example, metallic fins are often attached to the IC, and cooling fans are provided to enhance air flow over the IC. However, when the device is under test, the device is not encapsulated and, typically, its substrate is thinned down for testing purposes. Consequently, no means for heat dissipation are available and the device under test (DUT) may operate under excessive heat so as to distort the tests, and may ultimately fail prematurely. Therefore, there is a need for effective thermal management of the DUT.
One prior art system used to cool the DUT is depicted in FIG. 1a. The cooling device 100 consists of a cooling plate 110 having a window 135 to enable optical probing of the DUT. The window 135 may be a simple cut out, or may be made of thermally conductive transparent material, such as synthetic diamond. The use of synthetic diamond to enhance cooling is described in, for example, U.S. Pat. No. 5,070,040, which is incorporated herein by reference. Such a solid transparent window is often referred to as a transparent heat spreader. Conduits 120 are affixed to the cooling plate 110 for circulation of cooling liquid. Alternatively, the conduits may be formed as an integral part of the plate, see, e.g., U.S. Pat. No. 6,140,141.
FIG. 1a depicts in broken line a microscope objective 105 used for the optical inspection, and situated in alignment with the window 135. During testing, the cooling plate is placed on the exposed surface of the DUT 160, with the window 135 placed over the location of interest. When the cooling plate 110 is used with a transparent heat spreader 135, an oil layer, or other high index of refraction fluid, is sometimes provided between the transparent heat spreader 135 and the DUT 160 in order to improve the optical coupling from the DUT 160 to the transparent heat spreader 135. Heat from the device is conducted by the cooling plate to the conduits and the cooling liquid. The cooling liquid is then made to circulate through a liquid temperature conditioning system, such as a chiller, thereby removing the heat from the device. Typically, however, the DUT includes auxiliary devices 165, which limit the available motion of the cooling plate, thereby limiting the area available for probing. To overcome this, custom plates are made for specific devices, leading to increased cost and complexity of operation of the tester.
Another problem with the conventional cooling plate is insufficient and non-uniform heat removal from the DUT. FIGS. 1b and 1c schematically show a conventional cooling plate with a transparent heat spreader of a somewhat modified design from that of FIG. 1a. FIG. 1b is a top view, while FIG. 1c is a partial cross section along lines Axe2x80x94A in FIG. 1b. A transparent heat spreader 110xe2x80x2 is soldered to a frame 130 using, for example, an indium solder at interface 115. A DUT (not shown) is observable through transparent heat spreader 110xe2x80x2, and oil or other fluid may be provided between the DUT and the heat spreader. The frame 130 is attached to, or is formed as an integral part of, an inner metallic heat sink 140 which, in turn, is attached to an outer metallic heat sink 150. Conventionally, the inner metallic heat sink 140 is attached to the outer metallic heat sink 150 using screws and having no heat conducting material there-between. Chilled air is pumped through inlet 170 to circulate through the outer metallic heat sink 150, and is exhausted through outlet 175.
As can be understood, heat is transported from the DUT to the transparent heat spreader 110xe2x80x2, to the frame 130, to the inner metallic heat sink 140, to the outer metallic heat sink 150, and to the chilled fluid. However, the interfaces between the various elements act to resist heat conduction, thereby reducing the efficiency of heat removal from the DUT. Additionally, the temperature gradient across the various elements encourages heat gain from the ambient. In fact, studies have shown that heat gain from the ambient can be greater than the heat removal from the DUT. The thermal resistance present in the heat conduction path, along with the significant heat gained from the ambient, combine to dramatically increase the difficulty in lowering the temperature of the transparent heat spreader and, thereby, lowering the temperature of the DUT.
Of particular interest to the present inventors is the temperature at the periphery of the transparent heat spreader (locations of 1-8 in FIG. 1b). That is, the inventors speculated that a system having efficient heat transport will lower the temperature at the periphery of the transparent heat spreader, and thereby the temperature of the heat spreader and the DUT. To investigate that, a temperature distribution of an industry standard semiconductor thermal test chip, cooled by transparent heat spreader as exemplified in FIG. 1b, was simulated using a Finite Element model. The model simulated the temperature distribution in the transparent heat spreader, as well as the heat conduction from the transparent heat spreader, across the indium solder, to the periphery of the inner metallic heat sink. Using a one dimensional heat conduction analysis to calculate the temperature rise from the surface of the transparent heat spreader to the chip, the chip""s temperature distribution, and its maximum temperature, were determined. The accuracy of the prediction of the Finite Element model is directly tied to the accuracy of the imposed boundary conditions. In this case, a key boundary condition is the temperature at the inner periphery of the inner metallic heat sink, i.e., at the indium solder contact area. As can be understood, the temperature at this periphery is dependent upon the heat removal efficiency of the entire assembly.
The impact of the boundary condition on the DUT""s maximum temperature can be determined from FIG. 2 (determined with the Finite Element model). FIG. 2 is a plot of the transparent window""s maximum temperature as a function of the boundary condition for various heat loads, wherein this maximum temperature drives the maximum DUT temperature. As is evident from FIG. 2, the lower the temperature at the periphery of the heat sink, the lower the maximum temperature of the transparent window, which will result in a lower maximum DUT temperature. That is, the inventors speculated that a system that can efficiently lower the temperature at the boundary of the transparent window, will also efficiently remove heat from the DUT.
To verify the accuracy of the model, extensive experiments were conducted with a test chip, using a cooling plate assembly similar to that of FIG. 1b. The DUT was powered to various heat fluxes, while being cooled by the cooling plate in a conventional manner. The cooling plate was instrumented for temperature measurements at the locations marked 1-18 in FIG. 1b, and the results for a chip powered at 20 W/cm2 are shown in FIG. 3 (data are provided in degrees Centigrade). The results at the periphery of the transparent heat spreader were used as the boundary condition for further Finite Element model analysis. The model was executed and a prediction was made of the temperature distribution in the transparent window, as well as its maximum temperature. The maximum DUT temperature was then calculated as a function of the oil layer thickness and plotted in FIG. 4. For a measured oil layer thickness of 80 xcexcm, the results of FIG. 4 correspond with the experimentally measured temperatures of the test DUT, thereby verifying the accuracy of the model.
As can be readily understood from the above discussion there is a need for an innovative, inexpensive, flexible, and thermally effective thermal management solution for chip testers or probers.
The results of the investigation detailed above highlight the importance of lowering the temperature at the periphery of the transparent window to the maximum extent possible. The present invention provides effective solutions for heat removal from the periphery of the transparent window, thereby providing a mechanism for removing heat from a DUT and allowing for inspection of the device under electrical stimulation. Therefore, the system is particularly adaptable for use with optical microscopes used for probing, diagnostics and failure analysis of the DUT.
In one aspect of the invention, a thermal management system is provided which utilizes a heat spreader for removing heat from the DUT and an atomized liquid spray system for removing heat from the heat spreader.
In another aspect of the invention an objective lens housing and a transparent cooling plate are placed inside a spray chamber. A spray cooling arrangement is provided to spray coolant onto the cooling plate. The spray chamber is sealed to a plate upon which the DUT is situated. The pressure inside the chamber may be controlled to obtain the proper evaporation of the sprayed coolant. Pressure transducers and temperature sensors may be installed on the pressure chamber to monitor the operation of the thermal management system.
In another aspect of the invention, the spray cooling is accomplished using several banks of atomizers to cool the periphery of the transparent heat spreader. According to one implementation, all of the atomizers are commonly connected to one liquid supply. On the other hand, according to other implementations, liquid delivery to each, or to groups, of atomizers may be controlled separately so as to vary the pressure, the timing, and/or the type of liquid delivered to various atomizers.
In yet another aspect of the invention, a cooling plate is soldered onto a holder. The holder is used to press the cooling plate against the DUT. Several atomizers are provided for spraying cooling fluid onto the periphery of the cooling plate. The holder may additionally serve to prevent the sprayed fluid from reaching the central part of the cooling plate, so as not to obscure the optical path.
In a further aspect of the invention, a cooling plate is soldered onto a holder. The holder is used to press the cooling plate against the DUT. The holder is provided with a hollow cavity, inside which the atomizers are a fixed. The atomizers spray cooling liquid onto the upper interior part of the holder, and the sprayed liquid is then evacuated via the hollow cavity inside the holder. In this manner, no liquid reaches the cooling plate; rather, heat is removed from the cooling plate via the cooled holder.
According to a further aspect of the invention, a DUT is affixed onto a pc board and the cooling plate is provided over the DUT. A metallic clamp holds the cooling plate and the DUT onto the pc board. An indium gasket may be provided between the metallic clamp and the cooling plate. A hollow holder is then pressed against the metallic clamp. Another indium gasket may be placed between the metallic gasket and the holder. The holder is provided with hollow injection chamber, inside which the atomizers are affixed, and hollow return chamber for collecting sprayed liquid. The atomizers spray cooling liquid onto the upper interior part of the holder, and the sprayed liquid is then evacuated via the hollow return chamber inside the holder. In this manner, no liquid reaches the cooling plate; rather, heat is removed from the cooling plate via the cooled holder.
According to yet another aspect of the invention, a transparent heat spreader is movably attached to the objective assembly via a holder. The holder may slide freely, be spring loaded, or flexibly mounted to the objective assembly. This arrangement is provided so that once the heat spreader is placed against the DUT, the objective assembly may be moved further in order to reach appropriate focus point. Coolant is delivered to the spray heads, which deliver coolant spray onto the heat spreader or, optionally, also onto the DUT itself.
According to yet another aspect of the invention, a transparent heat spreader is provided having both a cooling channels and a cooling spray.
In a further aspect of the invention, control instrumentation is provided for accurate operation of the thermal management system. The DUT temperature can be controlled via coolant temperature, coolant flow rate (directly tied to coolant delivery pressure), and coolant boiling point (a function of spray chamber pressure and vapor temperature. Note that at its saturation temperature, the temperature of the saturation liquid is the same as its vapor (non-superheated)). A temperature sensor close to the coolant delivery point monitors the coolant delivery temperature, which is fed back to the thermal management system""s controller. The controller controls a liquid temperature conditioning system, which may be a chiller or other device to control the coolant""s temperature to a pre-determined value. Such systems are well known to those skilled in the art.
Spray chamber pressure is measured with a pressure transducer in communication with the spray chamber. Vapor temperature (measured with a temperature sensor in communication with the spray chamber) and spray chamber pressure determine the coolant""s boiling point, which in turn influences the manner in which the DUT temperature is controlled (via the transparent cooling plate). The spray chamber pressure can be manipulated to influence the coolant""s boiling point. The spray chamber pressure may be affected, for example, by a solenoid valve in communication with the spray chamber, by adjusting the return pump""s speed, or by manipulating the pressure inside the liquid temperature conditioning system""s reservoir. A mechanical pressure relief valve provides a safety release in the event that the solenoid valve fails.
One or more of the afore-mentioned approaches, individually or in combination, may be used to control the coolant flow rate and/or the coolant""s boiling point. The ultimate goal is to use the instrumentation to control the DUT to a pre-determined temperature. The temperature of the DUT may be measured by mechanical contact with a thermocouple or other sensor, by non-contact means such as a thermal imaging camera, or by any other means suitably accurate for the intended temperature stability. Any means for measuring the DUT temperature may be employed in the control of the DUT temperature. The specific examples given here are meant for illustrative purposes only and are not meant to limit this invention in any way.
A computer or other electronic or mechanical control system may be used to monitor DUT temperature and provide the necessary adjustment of spray. For example, if the DUT temperature rises, the computer could increase the flow rate, decrease the fluid temperature, or both.
The terms xe2x80x9ctransparent windowxe2x80x9d and xe2x80x9ctransparent heat spreaderxe2x80x9d are used herein somewhat interchangeably. As can be appreciated, the element functions as a window for the optical system, and as a heat spreader for the thermal management system. Also, the term xe2x80x9ctransparentxe2x80x9d is used herein loosely. That is, as can be appreciated, a window may be transparent to a system operating a one wavelength, while opaque for system operating at another wavelength. For example, a xe2x80x9ctransparentxe2x80x9d window for a system operating at the IR range, may be opaque at the visible range. Therefore, when used herein, the term xe2x80x9ctransparentxe2x80x9d means transparent for the wavelength of interest.
The invention further comprises a method for controlling the temperature of an integrated circuit (IC) undergoing diagnostics, the method comprising: attaching the IC to a socket; providing a transparent heat spreader over the IC; and injecting coolant onto the transparent heat spreader from at least one spray head.